1. Field of the Invention
The present invention relates to a digital watermark-embedding apparatus, a digital watermark-embedding method, and a medium having recorded therein a program for practicing the digital watermark-embedding method. In particular, the present invention relates to a method for regulating the intensity that is required to embed digital watermarks into a digital image.
2. Description of the Related Art
With digital content such as digitized audio and digitized video data, it is easy to provide a faithful reproduction of original content. Therefore, it is a recent issue of importance to protect the copyright of the digital content. A “digital watermark” is used as one solution to the issue.
The digital watermark is an art of embedding data into video data in such a manner as to prevent human beings from perceiving degradation in image quality, and of detecting the embedded data from such image data.
The digital watermarks embedded in the image data must properly be detected from pictures that have been subjected to image processing such as image compression and noise addition. A degree to which the digital watermarks withstand such detection after the image processing is herein called toughness-with-image-processing.
The image quality degradation and the toughness-with-image-processing form a trade-off relationship, and are balanced with one another by adjusting a value of embedment intensity.
For example, an increased value of the embedment intensity enhances the toughness-with-image-processing while increasing degradation in image quality of an image having the digital watermarks embedded therein. Meanwhile, a decreased value of the embedment intensity inhibits the image quality degradation, but reduces the toughness-with-image-processing.
A prior art digital watermark-embedding method designed to automatically regulate such a trade-off relationship has been taught as an illustration in published Japanese Patent Application Laid-Open No. 11-346302. This method is now described in brief.
FIG. 4 is a block diagram illustrating a prior art digital watermark-embedding system. In FIG. 4, a category-classifying unit 401 calculates a characteristic amount of an input image, and then determines, from the calculated characteristic amount, a category under which the image falls. The category-classifying unit 401 feeds a category index to an accumulation apparatus 402.
The accumulation apparatus 402 has a digital watermark characteristic table for each of the category indexes, and selects a proper characteristic table in accordance with the category index sent from the category-classifying unit 401. The digital watermark characteristic table describes a relationship between digital watermark intensity, a degree of degradation in image quality, and an evaluation of toughness with attack (image processing).
A digital watermark intensity-calculating unit 403 feeds the digital watermark intensity into the accumulation apparatus 402, and then calculates an optimum digital watermark intensity using the degree of degradation in image quality, the evaluation of toughness, and constraint information. The degree of degradation in image quality and the evaluation of toughness are fed from the accumulation apparatus 402, while the constraint information is entered from a user. The digital watermark intensity-calculating unit 403 delivers the calculated optimum digital watermark intensity to a digital watermark-embedding unit 404.
The digital watermark-embedding unit 404 converts embedded data into digital watermarks, and then embeds the digital watermarks into the input image at the optimum digital watermark intensity that is fed from the digital watermark intensity-calculating unit 403. As a result, a digital watermark-embedded image is produced.
(1) The prior art employs activity (a squared average value of AC frequency components) of the entire image as a characteristic amount of the input image.
However, the input image is not always uniformly formed. In nearly all of the input images, more diversified, complicated regions and less diversified, simple regions are scattered.
An excess degree of intensity, at which the digital watermarks are embedded into the image, renders digital watermark-caused image quality deterioration more conspicuous. In particular, such deterioration in image quality becomes more pronounced at less diversified regions of the input image.
As a result, the prior art has a problem in that the digital watermarks are often embedded at an improper degree of intensity over local regions of the input image, with a consequential degradation in image quality.
(2) The degree of degradation in image quality according to the prior art is a numeric value calculated in accordance with the input image and the digital watermark-embedded image that is obtained immediately after the digital watermarks are embedded into the image.
Now, assume that video data accumulated and saved in a recoding medium such as a hard disk are transmitted and distributed through a network. In this instance, in order to reduce an amount of data, video data-forming images are encoded in accordance with, e.g., a MPEG system for moving pictures, or otherwise, e.g., a JPEG system for static images.
As a result, a pre-encoded, digital watermark-embedded image usually differs from a post-encoded, digital watermark-embedded image.
For example, in a digital watermark-embedded image that has been compressed according to a low compression bit rate, compression-caused block distortions are noticeably observed. Therefore, such a digital watermark-embedded image is considerably poorer in image quality than a pre-compressed, digital watermark-embedded image.
In short, the prior art takes no account of a change in image quality between pre-compression and post-compression. This causes a further problem in that the digital watermarks cannot be embedded into the input image at a preferable degree of embedment intensity, with a consequentially improper relationship between degradation in image quality and the toughness-with-image-processing.